1. Field of the Invention
The present invention relates to a planar type speaker provided with a diaphragm (i.e., vibrating plate), and more particularly to the planar type speaker and a speaker system using the speaker, wherein the diaphragm is of a planar type.
2. Description of the Related Art
One of general type diaphragms used in a conventional speaker assumes a conical shape as a whole. In the prior art, there is also another general type diaphragm, which assumes a planar shape used in a planar type speaker. A conventional one of the planer type speakers is disclosed, for example, in Japanese Utility Model Publication No. S57-12634 (Publication date: 1982).
In the planar type speaker of this Japanese Utility Model Publication, as shown in FIGS. 7 and 8, a diaphragm 31 is constructed of a flat plate. Fixedly mounted on a rear surface of the diaphragm 31 are a plurality of coil bobbins 33. Wound around each of these coil bobbins 33 is a piece of wire to form a voice coil 32. The diaphragm 31 is surrounded by a frame 35 and supported on each side by this frame 35 through a flexible membrane piece (hereinafter referred to simply as the “edge member” 34). The edge member 34 assumes a semicircular shape in cross section, and encircles the entire outer peripheral edge of the diaphragm 31.
The edge member 34 is provided with a notched hole 36 in each of its four corner portions to permit the diaphragm 31 to displace or move vertically relative to the frame 35, as viewed in FIG. 8.
In the conventional planar type speaker, as described above, the notched hole 36 is provided in each of the four corner portions of the edge member 34 thereby helping the diaphragm 31 vertically displace or move relative to the frame 35. However, these notched holes 36 of the edge member 34 permit an air mass confined in a speaker enclosure (hereinafter referred to as the “enclosure air”) to escape from the enclosure. Such escape of the air passing through the notched holes 36 of the edge member 34 results in a distortion of the diaphragm 31, which causes a phase shift to impair the speaker's performance, particularly, in fidelity in sound reproduction thereof.
In the conventional speaker, as shown in FIG. 8, the voice coil 32 has a cylindrical shape. Such ordinary cylindrical voice coil itself merely offers a slight inductance in operation, in addition to a d-c resistance of wire of the voice coil. Further, as for the frequency characteristics, there is not any remarkable change in the voice coil. However, once the voice coil is put in a magnetic gap (i.e., an air gap in a magnetic circuit) and fixedly mounted on the diaphragm or like vibrating member, the characteristics of the voice coil drastically change in operation. In other words, when an acoustic signal is fed to the voice coil, an alternating magnetic force is produced in the voice coil so that the voice coil is magnetically moved. As the voice coil moves, a corresponding back electromotive force (back-emf) is produced in the voice coil so that an impedance of the voice coil increases in operation.
The impedance characteristic of the voice coil is very shape at the beginning of its graph to assume a crest-like form (at a lowest resonance frequency (Hz) f0), which is followed by a simple ascending slope. In the past, in order to flatten the impedance characteristic of the voice coil, an impedance correction circuit is employed.